Hacking nature: the tower bends with rising movement.

Two Predictions about the Distribution of Life in the Universe

There is a rule in software development called the zero one infinity rule, or ZOI, originally formulated by Willem van der Poel, according to which a process should be completely disallowed, or allowed only once, or no limit should be placed on the number of times it occurs. This is a rule of thumb for programming, but it also can be interpreted in a much wider sense. Robert Hazen, despite being the careful scientist that he is, gives a metaphysical spin to the zero one infinity rule: 

“In short, ‘zero, one, many’ means either that a natural phenomenon never happens (time running backward, for example), or it happens exactly once (a ‘singularity’ like the Big Bang), or it has happened more times than we can count (perhaps the origin of life).” (Robert Hazen, Symphony in C: Carbon and the Evolution of (Almost) Everything, p. 199)

In the context of the zero one infinity rule, we know that the appearance of life in the universe does not exemplify zero, so either life in our universe is one or many. Taking each the horns of this dilemma in turn, if life on Earth is unique, there is nothing more to say about the distribution of life in the universe: it has a distribution of one, limited to Earth. Any interesting prediction regarding the distribution of life in the universe will either be a prediction of whether our universe exemplifies one or many, or it would be a prediction regarding the distribution of the many instances of life in the universe. My prediction will be concerned with the latter, not the former; I am not going to predict how much life there is in the universe, or the frequency of its appearance in space or time.

If we were to establish that life is unique and distinctive to Earth, there would be as little to say as there would be to say of a universe with zero life, so we begin with the hypothesis that life is sufficiently common in the universe that there are enough instances of independent origins of life events that statistically valid generalizations can be made regarding its distribution. I adopt this hypothesis not because I believe it to be true (at present I have no reason to prefer the hypothesis of the uniqueness of terrestrial life or the prevalence of life in the universe), but only because it is a useful point of departure for thinking about life in the universe.

Supposing, then, that there are many worlds with life, and further supposing that life on these many worlds independently originated (or, at least, mostly independently originated, meaning that panspermia plays little or no role in the large-scale distribution of life in the cosmos, but more on panspermia below), my prediction on these assumptions concerns the distribution of mechanisms by which life arises in this scenario of life being prevalent in the universe.

Suppose further that we make a complete survey of the possible biochemical pathways to life, of which there are many already, and, we can infer, there will be more as origins of life research continues to develop. Again, I cite Robert Hazen, who gave a comic twist to the number of origins of life scenarios that contend for researcher’s attention:

“A popular game in origins-of-life research is to dream up an ‘origins scenario’—an elaborate, sweeping, often untestable story of chemical and physical circumstances by which the living world emerged from a lifeless geochemical milieu.” (Op. cit.)

Given many possible origins of life biochemical pathways, and many worlds with biospheres, it would be reasonable to assume that different biochemical pathways are responsible for the origins of life on different worlds. This is a reasonable assumption, but not a necessary assumption. We do not yet know how tightly constrained life is, but if life is tightly constrained, there may be only a single biochemical pathway to a single kind of life. If this is the case, I predict, on this basis, that life on Earth will be shown to be distinct. (This is yet another prediction, distinct from the two predictions made below, thus not included in the two predictions noted in the subtitle above.) However, there could be a single biochemical pathway to life that is represented on multiple planets throughout the universe.

If life is not tightly constrained, that is to say, if life is loosely constrained, or unconstrained and prolific in the universe, then many biochemical pathways to life will be represented on many different worlds. What kind of distribution of origins of life mechanisms would we expect to see under these circumstances? My two predictions for this distribution are based on two familiar ideas: the bell curve and the Pareto principle: 

Origins of Life Bell Curve: I predict of the many     biochemical pathways to life, that the bump in the bell curve will be     filled with the most common mechanisms for the origins of life,     representing some degree of mediocrity of the complexity of the process;     the left of the bell curve will be sparsely populated by the simplest possible     mechanisms, while the right of the bell curve will be sparsely populated     by the most complex mechanisms for the origins of life.  

Origins of Life Pareto Principle: I further predict that     about 20 percent of the biochemical pathways to life will account for     about 80 percent of instances of planets with biospheres, making these 20     percent of mechanisms literally the vital few, i.e., those     mechanisms most responsible for life’s prevalence in the universe.  

I furthermore predict that terrestrial life will exemplify the principle of mediocrity, such that the mechanisms responsible for origins of life on Earth will fall close to the center of the bell curve, and these mechanisms will represent those 20 percent of such mechanisms responsible for 80 percent of life in the universe. In other words, life on Earth is in the 80 percent, and therefore typical of life in the universe. 

There is much more that could be done to clarify the above predictions, and as further origins of life biochemical pathways are identified and refined, it will become increasingly possible to compare and contrast these mechanisms, and eventually to quantify the degree of complexity of each. The degree of complexity of a biochemical pathway to life is itself an idea in need of clarification. What are the dimensions of complexity of origins of life biochemical pathways? These may include the number of steps necessary to pass from inert chemical reactions to biochemistry, the specific mineralogical prerequisites for biochemistry (and how many steps are necessary for these precursors to evolve), the number and diversity of chemical processes that are required, the amount of time necessary for these processes to converge upon life, and so on.  

In the illustration above I have identified the most complex origins of life pathway to be panspermia, but this is ambiguous. However, my reason for doing so is that any origins of life on a planet (or other celestial body, such as a moon) in which panspermia is the mechanism necessarily involves some other origins of life biochemical pathway plus the extra step of a panspermatological vector. It could be argued that a simple biochemical pathway plus panspermatological distribution is likely to be simpler than the most complex biochemical pathways that do not involve panspermia (i.e., all steps of which occur on a single planetary body). This could be factored into a more adequate and refined quantification of the complexity of biochemical pathways to the origins of life. 

Needless to say, I will not live to see my predictions either confirmed or disconfirmed. Any survey of life in the universe, even a superficial and perfunctory survey, would require cosmological scales of time to investigate a cosmos filled with potentially inhabited worlds. Also, the effort to confirm or disconfirm such predictions is predicated upon a scientific effort that would also have to be cosmological in scale, and even if life is prevalent in the universe, there is no assurance that intelligent agents descended from any biosphere would take up this task at the requisite scale.

There is a slight possibility that our solar system is filled with microbial life in all manner of unlikely niches, and, if this is the case, and if we were to discover our solar system not only to be rich in life, but also that this life was the result of independent origins, then we could extrapolate from life in our solar system to the mechanisms of the origins of life represented in the wider universe. In this case, our solar system would be a cosmological Petri dish, and it might well be possible that I could see the first results of exploration of our solar system, whether through sample return missions or through boots-on-the-ground research. I make these predictions, then, not with an eye toward being proved right or wrong, but out of disinterested curiosity in what we might call stochastic metaphysics—that is to say, how frequency distributions ought to predict the ultimate constitution of the natural world. Thus naturalistic metaphysics can be speculative as well as descriptive.  

In the event that life from multiple origins events is to be found throughout our solar system, the extrapolation of the distribution of its origins of life mechanisms to the wider universe itself would constitute a further prediction: that the distribution of origins of life mechanisms in the small (in our solar system) will be mirrored by origins of life mechanisms in the large (in the universe). I hesitate to endorse this prediction, as the chemical compositions of other planetary systems derived from other proto-planetary discs, and these proto-planetary discs in turn derived from distinct precursor events (viz. the particular chemical composition of supernova events in the stellar neighborhood that would enrich proto-planetary discs with their elements), will be sufficiently distinct from the chemical composition of our solar system that the different abundances of elements and isotopes will likely beget different chemistries.  

To recap: I said I would make two predictions about the distribution of life in the universe, but I actually made four predictions: (1) if life is tightly constrained, Earth will be the only living world, (2) the origins of life bell curve, (3) the origins of life Pareto principle, and (4) life on Earth will exemplify the principle of mediocrity based on the distribution of life predicted above. I also suggested a fifth prediction that could be made, but I hedged on that one.